Method in the production of iron powder of desired grain size



Nov. 11, 1958 N. H. BRUNDIN METHOD IN THE PRODUCTION OF IRON POWDER OFDESIRED GRAIN SIZE Filed. Feb. 2o, 195

llamas/f mi llinys InvehTor METHOD TN THE PRODUCTION OF IRON POWDER OFDESIRED GRAIN SIZE Nils H. Brundin, Hoganas, Sweden, assignor toHoganas- Billesholms Aktiebolag, Hoganas, Sweden, 21 corporation ofweden Application February 20, 1951, Serial No. 211,982

3 Claims. (Cl. 75-.5)

When sponge iron is produced from oxide in powder form, especially ifcoke or charcoal is used as reducing agent, the temperature must, inpractice, be so high that the sponge iron is obtained as hard, sinteredcakes of considerable strength. These cakes can, after crushing, beground to a powder. Such sponge iron powder can be used for powdermetallurgical purposes if certain conditions are fullfilled. These are:

(1) The powder must pass a 100 mesh screen.

(2) The volume weight must not exceed 2.5, which means that the millingmust be done in such a way that the grains substantially retain theirorginal porous structure.

Because of this latter reason the grinding must be done very carefullyand in practice it has been shown that some conventional mills are notsuitable for the purpose. Two types of mills which have been foundsuitable are the pan mill and the disintegrator in which the treatmentis mild enough to let the grains retain their porous structure aftermilling.

In practice it Was found, however, that it was not possible to grindsponge iron cakes in these mills so that all powder obtained wassuitable for use for powder metallurgical purposes. About of theobtained powder would not pass a 100 mesh screen and even if this 20%was recirculated it was not possible to reduce the size any further.Therefore this 20% had to be used for other purposes. It is possible togrind this powder to a finer grain size in a ball mill but even then thepowder is not suitable for use in the powder metallurgy because by thistreatment the volume weight becomes much higher than the allowed 2.5.

The purpose of this invention is to make possible a more completeconversion of the hard, sintered cakes to a powder suitable for use inpowder metallurgy and thus to decrease the up to now unavoidable loss insaid powder which will not pass a 100 mesh screen.

The invention is based on the discovery that the grain size of the ironoxide used as raw material for the sponge iron production has aconsiderable influence on the production of a powder of wanted grainsize. This is rather astonishing since the reduction has to be carriedout at a temperature considerably higher than the temperature at which acrystallization and grain growth of iron takes place, wherefore it wasto be expected that the original grain boundaries would disappear and bewithout influence on the grindability. Further each original grain has aconsiderable porosity which ought to decrease the mechanical strengthand make it possible to split it up in the grinding process.

According to the invention the iron oxide which is to be used for powderproduction is, by suitable means such as grinding or sieving, reduced toa grain size which is atent O "ice the same as or less than the grainsize desired in the iron powder, whereafter the oxide powder is reducedto sponge iron, which after cooling and crushing is milled to the wantedgrain size.

Example.-Magnetite concentrate was mixed with powdered charcoal in theproportions :33 and the mixture heated to 1000 C. where it was keptuntil the reduction was complete. After cooling the obtained sponge ironwas crushed in a jaw-crusher and milled in a disintegrator. Aftermilling the powder was screened on a 100 mesh sieve and the powder notpassing the sieve was milled again. This treatment was repeated severaltimes and the different fractions weighed.

One experiment was carried out with a magnetite concentrate passing 40mesh and one with concentrate passing 100 mesh.

The result is given in the accompanying drawing where the accumulativepercentage of powder passing 100 mesh is shown as a function of thenumber of milling operations.

As is seen from the drawing the unavoidable loss when using 40 meshconcentrate amounted to about 20 percent whereas the corresponding losswhen using 100 mesh concentrate as starting material only amounted toabout 2.5 percent.

We claim:

1. A method for the production of iron powder of the desired grain sizefor powder metallurgical purposes which comprises mechanically reducingan iron oxide ore to a particle size not greater than said desired grainsize, chemically reducing said iron oxide ore to iron powder at atemperature suflicient to produce spong iron in the form of hard,sintered cakes, cooling said cakes and comminuting them to a powder ofsaid desired grain size.

2. A method for the production of iron powder as defined in claim 1 inwhich the iron oxide ore is of a grain size capable of passing a 100mesh screen, is free of carbonizable liquid, is reduced in the presenceof a solid reducing agent and at least about 97.5% of the powderproduced by comminuting will pass a 100 mesh screen and has a volumeweight not greater than about 2.5.

3. A method for the production of iron powder of about 100 mesh andsuitable for powder metallurgy which comprises mechanically reducingiron ore of greater particle size to not greater than about 100 mesh,chemically reducing the resulting ore at a temperature sufiicient toproduce sponge iron in the form of a hard, sintered cake and cooling anddisintegrating said sponge iron cake to about 100 mesh.

References Cited in the file of this patent UNITED STATES PATENTS2,200,491 Cross et al. May 14, 1940 2,216,770 Drapeau et al Oct. 8, 19402,252,697 Brassert Aug. 19, 1941 2,279,013 Roseby Apr. 7, 1942 2,402,120Boegehold et al June 18, 1946 2,413,492 Firth Dec. 31, 1946 FOREIGNPATENTS 568,786 Great Britain Apr. 20, 1945 OTHER REFERENCES PowderMetallurgy, page 119. Edited by Wultf. Published in 1942 by the AmericanSociety for Metals, Cleveland, Ohio.

3. A METHOD FOR THE PRODUCTION OF IRON POWDER OF ABOUT 100 MESH ANDSUITABLE FOR POWER METALLURGY WHICH COMPRISES MECHANICALLY REDUCING IRONORE OF GREATER PARTICLE SIZE TO NOT GREATER THAN ABOUT 100 MESH,CHEMICALLY REDUCING THE RESULTING ORE AT A TEMPERATURE SUFFICIENT TOPRODUCE SPONGE IRON IN THE FORM OF A HARD, SINTERED CAKE